In the field of LED drivers for offline applications such as retrofit lamps, solutions are demanded to cope with high efficiency, high power density, long lifetime, high power factor and low cost, among other relevant features. While practically all existing solutions compromise one or the other requirement, it is essential that the proposed driver circuits properly condition the mains power into the form required by the LEDs while maintaining compliance with present and future power mains regulations. It is of critical importance to guarantee that the perceptible light flicker does not exceed a maximum value (preferably zero) when the power factor is maintained above a certain limit.
Further, in off-line converters, energy from the power mains often needs to be drawn synchronously in proportion to the supplied voltage waveform in order to achieve a high power factor and low harmonic distortion. Power converter architectures with an independent preconditioner stage are traditionally employed to best accomplish this task without compromising the proper form of the energy to be supplied to the load.
Typically, two series-connected power stages are employed to obtain a high power factor while keeping the output power constant throughout a mains cycle (or supply cycle, i.e. the cycle of the mains voltage or the supply voltage). In those architectures, the first stage shapes the mains current and the second stage performs the power conversion to the load.
Nonetheless, for reasons related to complexity and cost, simplified powertrain solutions are adopted known conventionally as single-stage, where either of the two stages may essentially not be incorporated. As a consequence of such simplification, the aforementioned requirements may be largely compromised and/or converter performance highly degraded, particularly in terms of size, reliability and lifetime. The latter is usually mainly attributed to the need for using a bulky low frequency storage capacitor in parallel to the load when constant output power delivery is to be guaranteed.
Single stage solutions are common in literature. One reference example is given in the work of Robert Erickson and Michael Madigan, entitled “Design of a simple high-power-factor rectifier based on the flyback converter”, IEEE Proceedings of the Applied Power Electronics Conferences and Expositions, 1990, pp. 792-801.
An intermediate solution, half-way between the two-stage and single-stage approaches, is the single-stage converter with integrated preconditioner. Such solutions can feature reduced component count and high power density while keeping compliance with both load and power mains requirements. Other embodiments with a single power converting stage allow high power factors (HPF) by means of integrating a boost converter operating in discontinuous conduction mode. These converters actually combine the above mentioned two power conversion stages.
Typically, high power factor operation of driver devices for driving a load, such as LED units comprising one or more LEDs, causes strong 100 Hz output current ripple even if large filter capacitors are used. These filters are almost ineffective if employed parallel to LED loads with steep IV (current versus voltage) characteristics (also called “diode characteristics”). It is known from electric vehicles and photovoltaic systems that dc/dc-converters between load and capacitor improve the exploitation of (super-) capacitors.
Q. Hu and R. Zane, “A 0.9 PF LED Driver with Small LED Current Ripple Based on Series-input Digitally-controlled Converter”, Proceeding of the APEC 2010, pp. 2314-2320 describe a two-stage LED driver using a bidirectional step down converter as second power stage, which connects the 120 Hz capacitor to the LED load, which also is connected to the output of the first power stage.